Dielectric fluids are insulating materials conventionally used in certain electrical equipment such as high-voltage cables and transformers, in which they are impregnated on a solid material that acts as an insulator, such as a polypropylene film, optionally combined with a layer of paper, referred to as a “mixed film-paper” material. In transformers, these fluids also act as heat-transfer fluids. Dielectric fluids are also used in capacitors, where they make it possible to separate the conductive armatures and to thus prevent them coming into contact with one another and generating a short circuit. It is thus possible to manufacture high-capacity capacitors having a reduced size.
Various dielectric fluids have been marketed for replacing pyralenes (PCBs), which are chlorinated organic compounds considered to be persistent organic pollutants having effects that are harmful to human health and to the environment. They are essentially complex mixtures of hydrocarbons obtained by refining mineral oils derived from petroleum.
Thus, document U.S. Pat. No. 4,523,044 discloses a dielectric fluid consisting of a mixture comprising predominantly benzyltoluene oligomers, and also a small proportion of ditolylphenylmethane oligomers. A mixture of monobenzyltoluene and dibenzyltoluene is also available from ARKEMA under the trade name Jarylec® C101.
Document U.S. Pat. No. 4,744,000 describes the use, in capacitors, of dielectric fluids comprising phenylxylylethane in combination with phenyltolylmethanes, in particular with a mixture of monobenzyltoluene and dibenzyltoluene, and this being in particular for the purpose of obtaining improved performance with regard to the discharge inception voltage.
Lastly, document U.S. Pat. No. 5,017,733 describes the low-temperature electrical insulating capacity of various mixtures of oils containing bicyclic aromatic compounds. However, the oils used in this document do not comprise dibenzyltoluene.
Although the fluids of the prior art have a resistance to high voltages, which is characterized by a high partial discharge inception threshold and by a high rate of extinction of these discharges, their low-temperature behavior is not always satisfactory. This hinders their use in cold climates in particular. Thus, these fluids customarily have too high a viscosity at low temperature, which negatively affects the rate of diffusion of the gases (in particular of hydrogen), produced following a concentration of the electric field in certain zones of the device comprising these fluids. This results in an increase in the saturation level of the gas in the fluid, which generates bubbles leading to a partial discharge. This may result in a breakdown or in any case reduce the service life of the electrical device. In addition, these dielectric fluids have a tendency to crystallize at low temperature, which is also detrimental to the efficient operation of the electrical devices containing them.
Consequently, it would be desirable to be able to have a composition having a sufficiently low viscosity at low temperature, and a sufficiently low pour point, in order to be able to be used as a dielectric fluid at temperatures below −40° C., in particular below −60° C.
In addition, the low viscosity of these fluids would also make it possible to use them as heat-transfer fluids, especially in transformers.
The Applicant has discovered that these requirements could be satisfied by a composition containing, in addition to a mixture of benzyltoluene and dibenzyltoluene, at least one other specific compound.